An approach to the examination of the structure of layered biological tissue can be found in the measurement of the reflectance of plane diffuse photon density waves in the near-infrared range. The objective of this project is to take phase resolved reflectance measurements from both phantoms and biological tissues at moduolation frequencies of up to 2 GHz and to show the validity of the diffusion model to describe the measured phenomena. The technique allows the investigation of the structure of tissue to depths of 4-8 mm. A medical application may be found in improved examination techniques for deep burns as the method allows the investigation of the tissue structure without physical contact to the surface.